This invention relates to silica glass and, in particular, to a method of preparing tubular silica glass.
Since the advent of the sol-gel method of making silica glass, it has been possible to make silica glass of greater purity than that which could have been made using prior art methods. The field of application of this improved purity silica glass is extremely wide and precise manufacturing methods, quality and configuration of the glass vary depending upon its intended end use.
For example, silica glass made by the sol-gel method has been widely used as crucibles or boards for manufacturing semi-conductor devices and as oven center tubes for defusing ovens. It is also used to make glass equipment such as test tubes for chemical use and cells for optical measurement. Additionally, silica glass made by the sol-gel method is widely used as a substrate for thin film transistors (TFT). As a result of the many applications to which silica glass made by the sol-gel method has been and continues to be put, demand for this glass is expected to expand significantly.
At present, optical fibers made of silica glass are used as a primary material in optical communication systems designed for transmission of large amounts of information. When manufacturing optical fibers, tubular silica glass is required, either as a starting material or as a means of adjusting the outer diameter of the completed fibers. The quality and dimensional precision of this tubular silica glass must meet very stringent requirements. Consequently, this glass is very expensive.
The prior art methods for manufacturing tubular silica glass (not necessarily for use in optical communication systems) include the following:
1. Washing and etching of natural quartz crystal;
2. Obtaining silicon oxide from high purity silicon tetrachloride or silicon tetrahydride; and
3. Etching natural silica sand.
All of these prior art methods have disadvantages. In each method, it is extremely difficult to obtain tubular silica glass that meets the quality and dimensional precision required for use in optical communication systems. Furthermore, treatment of the glass at extremely high temperature is an element of all of these methods and the cost of producing the glass is high as a result. Finally, the third method cannot be used to make optical fibers at all because of the low purity of the final product.
Recently, manufacture of tubular silica glass for use as optical fibers according to the sol-gel method has been attempted. The sol-gel method produces silica glass of high purity. Additionally, the glass is produced by relatively low temperature treatment, and therefore the manufacturing cost is relatively low. However, it is difficult to make large tubular glass articles by the conventional sol-gel method.
In a method introduced by Hitachi, Ltd. a solution of silicon alkoxide, water, alcohol, and a suitable solvent is placed into a cylindrical container having a removable center bar. The solution is gelled to form a gel. Before contraction of the gel begins, the center bar is removed. Then the gel is dried and sintered to obtain tubular silica glass. However, it is difficult to make large pieces of tubular glass without cracks and it is not possible to make a long tube of silica glass using this method.
E. M. Rabinovich and his colleagues have manufactured tubular silica glass using a sol-gel method with ultra-fine particles of silica. The ultra-fine particles of silica are gelled and sintered and then ground and suspended in water to provide a hydro-sol solution. The hydro-sol solution is placed in a cylindrical container having a center bar and gelled. After gelling, the center bar is removed in order to yield a tubular gel. The tubular gel is dried and sintered. The relatively large tubular silica glass obtained is reported to be 1.7 cm inner diameter, 2.3 cm outer diameter, and 25 cm length.
Using the Rabinovich method, fracturing or cracking of the sol in the gelling step or the gel in the sintering step is less frequent than that which occurs using the earlier described method. However, the dry gel obtained by the Rabinovich method includes large pores and fracturing and cracking does occur. Additionally, the method is disadvantageous because of the necessity of using two dispersing steps which results in a possibility of mixing impurities and of non-uniformity of the optical quality of the glass.
There are two reasons why large silica glass articles, including large tubular silica glass articles, are not be obtained using a conventional sol-gel method. When making the dry gel, the drying step is accompanied by a large contraction of the gel. Additionally, when sintering a dry gel, foaming occurs. Both phenomena cause cracking of the gel.
According to a study by Nogami and his colleagues, in order to prevent cracking from occurring at the time of sintering when using a sol-gel method with a silicon alkoxide containing sol as a starting material, it is necessary to prepare a porous dry gel having a great many relatively large pores of about 50 to 100 .ANG.. This is supported by the fact that the dry gel obtained by Rabinovich et al. using the ultra-fine particles of silica is unlikely to crack. Additionally, the dry gels obtained by hydrolyzing a silicon alkoxide with aqueous ammonia (such dry gels are much more porous than ones obtained by hydrolyzing with acid) are relatively free from cracking.
The above described methods all include the step of uniformly dispensing the silica particles in the liquid phase. In order to accomplish this, stirring, application of ultrasonic vibration and the like are generally used. However, a great deal of energy and long periods of time are required to disperse the powdery material uniformly in the solution. It is also difficult to obtain a solution wherein the dispersion is perfectly uniform. However, if an agglomeration remains in the solution and appears in the completed glass articles, this represents a defect which lowers the quality of the glass product. Additionally, commercially available fumed silica generally includes impurities, such as Al and Fe in amounts ranging from a few ppm to several hundred ppm. Thus, such silicas are not particularly well suited for use in preparing optical glass in accordance with the invention.
Accordingly, it is desirable to provide a method for preparing tubular silica glass articles of larger dimension and of higher quality than those which can be provided using prior art methods and which overcomes the disadvantages inherent in the prior art methods.